1. Field of the Invention
The present invention relates to a semiconductor laser and more particularly to a phase-shifted distributed feedback (DFB) semiconductor laser which is high in mode stability and utilized in a digital Optical transmission system.
2. Description of the Prior Art
In the conventional digital optical transmission system, a semiconductor laser called a .lambda./4 phase-shifted DFB semiconductor laser and high in a single-mode quality is utilized, wherein the phase of a diffraction grating is shifted by a half period at the center of a laser resonance cavity. The .lambda./4 phase-shift structure is a known structure and is described, for example, in "Semiconductor, Japan Society of Applied Physics(ed.), Ohmsha Ltd., 1994, pp. 272, FIG. 12--12".
A .lambda./4 phase-shifted DFB semiconductor laser has, as FIG. 5 shows a cross-sectional view thereof, a .lambda./4 phase-shift structure 45 by which the phase between a first grating 43 and a second grating 44 is shifted by a half period, at the center of the laser cavity. In such a structure, a side mode suppression ratio may be advantageously high, since the laser oscillation wavelength therein is equal to the Bragg wavelength .lambda..sub.B, determined by the grating period .LAMBDA. thereof, that is EQU .lambda..sub.B 32 2.LAMBDA.n.sub.eff
where n.sub.eff is the effective refractive index.
However, this structure has a problem that the forward lasing output power cannot be monitored by the backward lasing output power (tracking error), because the ratio of the forward to the backward lasing output power varies with the bias current. Further, there is another problem that wavelength shifts at the time of modulation (chirping) are large so that code errors may be brought about in the long distance transmission. These problems arise from the fact that, because the .lambda./4 phase-shift structure is located at the center of the laser cavity, the electric field in this phase-shift region becomes very strong. And, as the bias current is increased, the non-uniformity of the distribution of the internal electric field extremely increases. The resulting fluctuation of carrier distribution causes the difference of the refractive index changes along locations in the laser cavity.
Further, there is another problem that the conversion efficiency of the output light against the input current is low, since it becomes difficult for the light to go out as the output power due to the concentration of the electric field in the central region of the laser cavity.
In Japanese Patent Application Laid-open No. 025086/1990, an example structure to solve the above problems is described, wherein, instead of positioning a .lambda./4 phase shift structure at the center of the laser cavity, as FIG. 6(A) shows across-sectional view thereof, a second diffraction grating 54, a period of which differs slightly from a period of a first grating 53 and a third grating 55, is set in a central region of about 100 .mu.m of the cavity and thereby the phase between the gratings 53 and 55 in end sections is shifted by a total of .lambda./4. This structure is characterized by the flatter longitudinal intensity distribution of electric field in the cavity, in comparison with the .lambda./4 phase-shifted DFB semiconductor laser described above. However, in this structure, because grating periods are not constant within the cavity, the laser does not oscillate at Bragg wavelength, which causes a problem of a low stability in the lasing mode.